The invention relates generally to a method for reducing distortion of a turbine case due to thermal variations and, more particularly, for reducing distortion of a gas turbine case due to thermal variations.
Gas turbines include a rotor and rotating disks that are attached to the rotor. Airfoils (or blades) are positioned at the outer diameter of the disks. These components are surrounded by a case. A gap is present between the tips of the rotor airfoils and the case. If the gap is too small, the airfoils rub against the case causing extensive damage. However, if the gap is too large, turbine efficiency is degraded at a cost of millions of dollars, for an excess of a few millimeters, over the lifetime of the turbine.
Achievement of gas turbine efficiency is further complicated by the fact that tip clearances vary during operation of the turbine. Gas turbine operating conditions vary substantially, based on a combination of intentional and unexpected effects. For example, the operational thermal environment of a gas turbine is complex, including effects from surrounding hot and cold pipes and the combustion chambers. In addition, variations in the thermal environment surrounding the case create temperature gradients within the case. The temperature gradients cause thermal stresses that distort the case.
Although designed to have a circular cross section, distortion of the case due to thermal stresses during operation of the turbine produces a noncircular case cross section. The deviation from a circular cross section reduces the tip clearances, causing the airfoils to rub against the case. To avoid this undesirable outcome, the turbine must be designed with an increased nominal tip clearance in order to compensate for the anticipated mechanical distortion of the case. In particular, the nominal tip clearances must be selected to compensate for the largest possible case distortion due to the large variation in thermal operating conditions for the gas turbine. However, as noted above, large tip clearances decrease the efficiency of the turbine at a cost of millions of dollars, for an excess of a few millimeters, over the lifetime of the turbine.
One previous technique to reduce the tip clearances involved trial-and-error attempts to alter the design of the turbine, followed by conducting computer simulations or tests to determine whether the resulting case distortion and tip clearances satisfy the desired operating criteria. However, given the complex thermal environment of the turbine, design changes can be laborious and time consuming, requiring many iterations. Moreover, a design change may be beneficial under certain operating conditions, while degrading performance under others. For example, changing the design of certain hot pipes near the case may provide a more uniform temperature distribution in the steady state, but adversely affect the temperature distribution during transient conditions, such as during start-up, emergency trip, restart, or shut-down operations. Thus, in addition to being laborious and time consuming, this previous redesign technique can be ineffective.
Accordingly, it would be desirable to develop a method for reducing the distortion of a turbine case due to thermal variations. Such a method would advantageously facilitate the reduction of tip clearances for gas turbines. In addition, it would be desirable for the method to be able to target portions of the turbine case prone to distortion and operation cycles that give rise to distortion. It would further be desirable for the method to avoid the trial and error approach of the prior art methods and to reduce the repeated computer modeling relative to the prior art methods.
Briefly, in accordance with one embodiment of the present invention, a method for controlling distortion of a turbine case includes measuring a temperature distribution for the turbine case. The temperature distribution includes a plurality of thermal gradients. The method further includes modeling a number of thermal stresses on the turbine case induced by the thermal gradients, calculating an out of roundness index resulting from the thermal stresses on the turbine case, and comparing the out of roundness index with at least one distortion limit. The method further includes controlling the temperature distribution until the out of roundness index satisfies the distortion limit.
In accordance with another embodiment of the invention, a system for controlling distortion of a turbine case includes a thermal measurement system for measuring the temperature distribution for the turbine case. The system also includes a computer configured for modeling a number of thermal stresses on the turbine case induced by the thermal gradients, calculating an out of roundness index resulting from the thermal stresses, comparing the out of roundness index with at least one distortion limit, and controlling the temperature distribution until the out of roundness index satisfies the distortion limit.